Add configurable FFU build partition drive letters

Add System, Windows, and Recovery partition drive-letter settings to the Hyper-V Settings UI, config save/load, BuildFFUVM parameters, and sample/docs. Defaults remain S, W, and R.

Validate selected letters early, log validation failures for the UI, include drive-letter metadata in VHDX cache matching, and normalize legacy malformed cache values. This only affects FFU build-time partitioning; ApplyFFU deployment letters remain unchanged.

docs/appsscriptvariables.md

@@ -96,9 +96,11 @@ s{
   "Processors": 4,
   "ProductKey": "",
   "PromptExternalHardDiskMedia": true,
+  "RecoveryPartitionDriveLetter": "R",
   "RemoveApps": false,
   "RemoveFFU": false,
   "RemoveUpdates": false,
+  "SystemPartitionDriveLetter": "S",
   "Threads": 5,
   "UpdateADK": true,
   "UpdateEdge": true,
@@ -116,6 +118,7 @@ s{
   "VMLocation": "C:\\FFUDevelopment\\VM",
   "VMSwitchName": "External",
   "WindowsArch": "x64",
+  "WindowsPartitionDriveLetter": "W",
   "WindowsLang": "en-us",
   "WindowsRelease": 11,
   "WindowsSKU": "Pro",